The present invention relates to a tunable semiconductor laser adapted for laser oscillation at an optionally variable wavelength.
A GaAs-Ga.sub.1-x Al.sub.x As double heterostructure distributed feedback laser diode is known as one of semiconductor lasers for effecting single longitudinal mode oscillation developed for long-distance optical transmission. The laser diode includes an active layer formed with a grating provided by a multiplicity of parallel grooves at a given spacing. The laser diode effects stable oscillation at a selected wavelength .lambda. given by the following equation. EQU .lambda.=2n.LAMBDA./m (1)
where n is the refractive index of the active layer, .LAMBDA. is the spacing between the grooves of the grating, and m is the order of the Bragg diffraction. Equation (1) indicates that the wavelength .lambda. of the laser beam emitted is dependent on the spacing .LAMBDA. of the grating, hence stable oscillation, whereas the diode involves the problem that the oscillation wavelength can not be altered. The laser diode may be made wavelength-tunable by utilizing the fact that the refractive index n varies with temperature, but to vary the temperature over a wide range requires a heating or cooling device, while there aries the need to use a constant-temperature chamber or thermostat to maintain the diode at the desired temperature stably. The use of such a device is undesirable when providing the diode in the form of an integrated unit including an optical circuit and other elements. Further because the Ga.sub.1-x Al.sub.x As cladding layer must be grown after forming the grating on the active layer, the diode is difficult to fabricate and likely to involve defects at the junction of the active layer and the cladding layer.